Combined planar and parabolic double reflector



Nov. 1, 1966 K. s. KELLEHER 3,283,331

COMBINED PLANAR AND PARABOLIC DOUBLE REFLECTOR Filed Sept. 24, 1965 2 Sheets-Sheet 1 INVENT OR KENNETH S. KELLEHER BY My ATTORNEY Nov. 1, 1966 K. s. KELLEHER 3,233,331

COMBINED PLANAR AND PARABOLIC DOUBLE REFLECTOR Filed Sept. 24, 1965 2 Sheets-Sheet 2 United States Patent 3,283,331 COMBINED PLANAR AND PARABOLIC DOUBLE REFLECTOR Kenneth S. Kelleher, 1115 Marine Drive, Alexandria, Va. Filed Sept. 24, 1965, Ser. No. 489,952 4 Claims. '(Cl. 343837) The present invention relates to a compact, low-noise microwave antenna assembly and is a continuation-in-v part of Serial No. 161,081, filed December 21, 1961, for Antenna Assembly.

In the microwave antenna art, some of the problems which constantly arise are: (1) noise caused by heat energy from the earth; (2) undesired side lobes which appear in the radiation pattern; and (3) feed blocking of the reflected waves. Feed blocking may be explained as follows: If a symmetrical paraboloid of revolution is used as the reflector, and a feed horn or the like is provided at the focus of the parabola, then there will be blocking of some of the waves which are reflected from the paraboloid of revolution and this disadvantageouslyaffects the radiation pattern, especially since this blocking occurs in the center of the wave path. There have been attempts in the past to correct this condition.

One type of low-noise antenna for microwave transmission is the shielded horn of the type disclosed by Kenneth S. Kelleher on pages 12-14 of the Antenna Engineering Handbook published by the McGraw-Hill Book Company, Inc. This is a horn paraboloid reflector system, also called a shielded horn, and is a very successful system which provides a partial solution to problems (1) and (2), and which is useful in reducing wide-angle radiation. Another beneficial feature of this system is that the feed point FP at the focus of the parabola R in no Way interferes with the reflected waves which are directed through radiation aperture A from the parabolic surface, so that there is no feed blocking.

However, this shielded horn is subject to the disadvantage of being fairly large in size. Such a horn must be at least as great inlength as the aperture dimensions. Although a wide flare angle may be used in an attempt to reduce the length of the born, when this is done, poor pattern characteristics follow. Therefore, while the low-noise characteristic of such an antenna assembly is desirable, its large size presents a disadvantage.

Accordingly it is an object of the present invention to reduce drasticallly the size of present low-noise antenna systems.

A further object of this invention is to provide an antenna assembly of the type described which provides a relatively low level of noise, yet is compact.

Another object is to provide an antenna assembly which substantially reduces noise caused by heat energy from the earth.

Still another object is to provide an antenna assembly which effectively lessens spill over and undesired side lobes which detract from the radiation pattern.

Yet a further object of the invention is to substantially eliminate feed blocking.

These objects and other objects of the invention are realized in a preferred embodiment of the invention wherein two reflectors are situated so that there will be substantially no blocking of the waves between the feed source and the finally directed wave path. The feed source is disposed on one reflector and directed by a horn or the like toward the other reflector. reflects the waves back in such a manner that they are reflected from the first reflector in a parallel arrangement so that the antenna may be directed or aimed as desired. Instead of using a reflector which forms a symmetrical parabola with respect to the feed point, only a The second reflector ice portion of one side of the parabola is used so as to eliminate feed blocking. Another reflector, which is planar, faces this arcuate portion of the parabolic reflector.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which:

FIG. 1 is a diagrammatic view illustrating the principle of operation of 'the invention described and claimed in Serial No. 161,018 in which parabolic and hyperbolic reflectors are combined in an antenna assembly;

FIG. 2 is a diagrammatic view of the geometrical arrangement of the reflectors of the present invention wherein the hyperbolic surface of FIG. 1 has been replaced by a planar surface and the antenna feed is located at the edge of the parabolic surface;

FIG. 3 is a perspective view of an antenna constructed in accordance with the theory of operation set forth in FIG. 2;

FIG. 4 is a diagrammatic view similar to FIG. 2, but wherein the antenna feed is disposed intermediate of the two reflectors; and

FIG. 5 is a perspective view of an antenna constructed in accordance with the theory set forth in FIG. 4.

In the study, analysis and design of antennas, transmitting and receiving antennas are treated alike; that is, both types are considertd as transmitting antennas. It should therefore be noted that although the analysis which follows mayseem to indicate that the antenna of the present invention is a transmitting antenna, it is primarily a receiving antenna, even though it may be used for transmitting purposes with excellent results. In accordance with this understanding the feed source, or feed, associated with the antenna means a feed for the antenna in the case of transmitting, and a feed for the receiver in the case of receiving.

The reflectors employed in the antennas of the present invention should be such that, considering them as a receiving antenna for the moment, the parallel waves entering the aperture will be reflected from the first surface on which the feed source is located and directed toward the second reflector. The wavefront of the waves reflected from the second reflector should be spherical and focus to the feed source on the first reflector. Also, to prevent feed blocking, the first reflector should be asymmetrical.

FIG. 1 is a diagram indicating a geometrical arrangement which may be considered to be a cross section of an antenna system. In a typical reflector arrangement, a parabolic reflector is represented by curve P. A point feed source is located at the focal point F of the parabola. This parabola has a axis X which is parallel to the radiating direction, and a directrix (not shown) which is required in order to construct the parabola so that all points therealong are equidistant from focus F and lines which are at right angles to the directrix.

In order to reduce the size of this parabolic reflector P, a hyperbola H is constructed from upper and lower focal points F and S, which are the focus of the parabola P and the feed point, respectively. A construction line I is drawn between these focal points F and S, and a perpendicular bisector B is constructed. From this given information, the hyperbola H may be constructed. The hyperbola H intersects parabola P at point 0.

The line ab, which defines the plane of the radiating aperture, is at right angles to the parabolic axis X and passes through its focus F. At the same time, an imaginary line adjoining a and c of hyperbola H is disposed parallel to axis X. With such a construction, the waves emanating therefrom will be directional and parallel to axis X.

However, it should be noted that in order to obtain particular characteristics it may become necessary to deviate somewhat from this optimal arrangement. For example, imaginary line we may be disposed at an angle with respect to axis X in such manner that there is some blocking of Waves. This may be readily tolerated up to about 10% blocking. Also, since this blocking occurs at the side or edge of the wave path, it is not as objectionable as the type of feed iblocking mentioned hereinbefore which is at the center of the wave path.

Points 1 and 2 along hyperbolic reflector H have been chosen at random. Lines V1 and V2 are now drawn from focus F to points 1 and 2. If we now assume that point S is a feed source and feeds microwaves thereto, a wave A1 will be radiated as indicated and will arrive at hyperbolic reflector H at point 1 and be reflected therefrom along the line R1. This will then be reflected in a direction parallel to axis X along line W1. A similar result will be provided by a radiated wave A2 which is reflected as indicated by line R2 and is then emitted from the radiating aperture as indicated by line W2.

From the foregoing it may be seen that although the actual point of radiation is disposed along the parabolic reflector P between points c and b, the waves which are reflected from the hyperbolic reflector H, R1 and R2, will actually be extensions of and coincident with lines V1 and V2. It will thus appear that these Wave have originally emanated from focus F of parabola P which may be considered the virtual feed source. Thus,,t-he system has all of the attributes of the typical parabolic reflector P, but is obviously drastically reduced in size.

FIG. 2 is a diagram similar to FIG. 1 in which the hyperbolic curve H has attained the extreme situation of becoming a straight line H. In this arrangement the source S is disposed at the lower edge of parabola P, and coincides with point b. The line H is the perpendicular 'bisector of line FS, which is the line connecting the two foci F and S, of the hyperbola H of FIG. 1. A hyperbola may be defined as the locus of a point which moves so that the difference of its distances from two foci is a constant. since every point on line H is equally distant from points F and S, and thereby, the difference of the distance from any point on lin'eH' from the points F and S is always zero.

FIG. 3 is a perspective view of one embodiment of the invention which incorporates a planar surface in combination with a parabolic surface. A curved plate of conducting sheet metal 10 is shaped to define a portion of a parabolic cylinder, which is the equivalent of the "portion of the curve in FIG. 2 shown between points b and 0. Panel 12 is also constructed of a conducting sheet metal material in planar form, and corresponds to the straight line portion H between points a and 'c of FIG. 2. Parallel end panels 14 and 16 join and support the edges'of panels'10 and 12. The front edges of the panels 10, 12, 14 and 16 are disposed in a common plane and define a radiating aperture 11.

A line source of radiation 18 is fed by transmission line 20 from a suitable microwave source 22.

The assembly is supported by means of a yoke 24, which may be revolved by means of motor 26. A similar motor arrangement (not shown) may be used to rotate the antenna assembly within the yoke as desired.

FIG. 4 is a diagram similar to FIG. 2, but in which the feed source S is located intermediate the parabola P and straight line H. If the line ab is assumed to be the plane of the radiating aperture, it will be appreciated that that portion of the parabola P which is located above point performs no useful function and may be eliminated.

I Accordingly, the embodiment of the invention shown in FIG. is constructed similarly to the embodiment of FIG. 3 except that the feed source 18 is located intermediate the parabolic surface and the planar surface Line H satisfies this requirement,

12, and planar surface .12 does not extend to intersect with parabolic surface 10. An additional closure panel 13 is provided to complete the mechanical antenna assembly.

While the invention has been shown and described with particular reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. A microwave antenna assembly for radiating microwaves in an intended direction comprising a first reflector of planar configuration defining a straight line in cross-section,

a radiation feed source disposed for radiating waves toward said first reflector; and

a second reflector adjacent said first reflector. and defining a parabolic arc in cross-section whose focus is disposed exterior-1y of said assembly;

said reflectors together defining a radiation aperture which is in a plane disposed at right angles to the intended radiation direction,

said straight line being the perpendicular .bisector of the line connecting said focus and said feed point,

said reflectors being arranged so that waves from said source directed toward said first reflector are reflected toward said second reflector along lines coincident with imaginary lines between the focus and said second reflector,

whereby all waves reflected from said second reflector will be directed in a direction at right angles to the plane of said radiation aperture.

2. The combination according to claim 1 wherein the focus of the parabolic arc of said second reflector is in the same plane as said radiation aperture.

3. A directional, low-noise microwave antenna assembly, comprising, in combination a first reflector of planar configuration defining a straight line in cross-section,

a second reflector adjacent said first reflector and .de-

fining in cross-section a parabolic are whose focus is disposed exteriorly of said assembly;

a radiation feed source disposed within said assembly;

and

a pair of end frame panelsdisposed in planes parallel to the intended radiation direction of said assembly,

said panels and said reflectors together defining a radiation aperture which is disposed in a plane at right angles to the intended radiation direction and in the same plane as the focus of the parabolic arc defining said second reflect-or,

said straight line being the perpendicular bisector of the line connecting said focus and said feed point,

said reflectors being arranged so that waves from said source directed toward said first reflector are reflected toward said second reflector along lines coincident with imaginary linesbetween the focus and said second reflector,

whereby all waves reflected from said first reflector will eventually be directed in a direction .at right angles to the plane of said radiation aperture,

4. The combination according to claim 3 wherein the focus of the parabolic arc of said second reflector is in the same plane as said radiation aperture.

1,128,952 9/1956 France.

HERMAN KARL SAALBACH, Primary Examiner.

E. LIEBERMAN, Examiner.

5/1961 Parmeggiani 343775' 

1. A MICROWAVE ANTENNA ASSEMBLY FOR RADIATING MICROWAVES IN AN INTENDED DIRECTION COMPRISING A FIRST REFLECTOR OF PLANAR CONFIGURATION DEFINING A STRAIGHT LINE IN CROSS-SECTION, A RADIATION FEED SOURCE DISPOSED FOR RADIATING WAVES TOWARD SAID FIRST REFLECTOR; AND A SECOND REFLECTOR ADJACENT SAID FIRST REFLECTOR AND DEFINING A PARABOLIC ARC IN CROSS-SECTION WHOSE FOCUS IS DISPOSED EXTERIORLY OF SAID ASSEMBLY; SAID REFLECTORS TOGETHER DEFINING A RADIATION APERTURE WHICH IS IN A PLANE DISPOSED AT RIGHT ANGLES TO THE INTENDED RADIATION DIRECTION, SAID STRAIGHT LINE BEING THE PERPENDICULAR BISECTOR OF THE LINE CONNECTING SAID FOCUS AND SAID FEED POINT, SAID REFLECTORS BEING ARRANGED SO THAT WAVES FROM SAID SOURCE DIRECTED TOWARD SAID FIRST REFLECTOR ARE REFLECTED TOWARD SAID SECOND REFLECTOR ALONG LINES COINCIDENT WITH IMAGINARY LINES BETWEEN THE FOCUS AND SAID SECOND REFLECTOR, WHEREBY ALL WAVES REFLECTED FROM SAID SECOND REFLECTOR WILL BE DIRECTED IN A DIRECTION AT RIGHT ANGLES TO THE PLANE OF SAID RADIATION APERTURE. 